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Bustling trade routes, urban growth and changes to the sex trade in the city of Kinshasa in the 1920s created a 'perfect storm' for the emergence of HIV pandemic that has infected almost 75 million people worldwide to date, scientists say.

An international team of scientists, led by University of Leuven in Belgium and Oxford University, has reconstructed the genetic history of the HIV-1 group M pandemic, the event that saw HIV spread across the African continent and around the world, and concluded that it originated in Kinshasa, the capital of the Democratic Republic of the Congo (DRC).

The team's analysis suggests that the common ancestor of group M is likely to have emerged in Kinshasa around 1920 (with 95 per cent of estimated dates between 1909 and 1930).

HIV is known to have been transmitted from primates and apes to humans at least 13 times but only one of these transmission events has led to a human pandemic.

The team's analysis suggests that, between the 1920s and 1950s, a 'perfect storm' of factors, including urban growth, strong railway links during Belgian colonial rule, and changes to the sex trade, combined to see HIV emerge from Kinshasa and spread across the globe.

"Until now most studies have taken a piecemeal approach to HIV's genetic history, looking at particular HIV genomes in particular locations," said Professor Oliver Pybus of Oxford's Department of Zoology, and senior author of the paper.

"For the first time we have analysed all the available evidence using the latest phylogeographic techniques, which enable us to statistically estimate where a virus comes from.

"This means we can say with a high degree of certainty where and when the HIV pandemic originated. It seems a combination of factors in Kinshasa in the early 20th Century created a 'perfect storm' for the emergence of HIV, leading to a generalised epidemic with unstoppable momentum that unrolled across sub-Saharan Africa," Pybus said.

One of the factors the team's analysis suggests was key to the HIV pandemic's origins was the DRC's transport links, in particular its railways, that made Kinshasa one of the best connected of all central African cities.

"Data from colonial archives tells us that by the end of 1940s over one million people were travelling through Kinshasa on the railways each year," said Dr Nuno Faria of Oxford University's Department of Zoology, first author of the paper.

"Our genetic data tells us that HIV very quickly spread across the Democratic Republic of the Congo (a country the size of Western Europe), travelling with people along railways and waterways to reach Mbuji-Mayi and Lubumbashi in the extreme South and Kisangani in the far North by the end of the 1930s and early 1950s.

"This helped establishing early secondary foci of HIV-1 transmission in regions that were well connected to southern and eastern African countries," Faria said.

The team suggests that, alongside transport, social changes such as the changing behaviour of sex workers, and public health initiatives against other diseases that led to the unsafe use of needles may have contributed to turning HIV into a full-blown epidemic.